The invention relates to an ignition circuit for vehicle.
An ignition circuit used on a vehicle is generally constructed so that switch contacts of an ignition key cylinder is connected in a power supply line and is closed or opened by a key which is specifically related to the cylinder.
A conventional ignition circuit is schematically shown in FIG. 8. The ignition circuit shown is of a point contact type which may be used with a four cylinder engine. Referring to FIG. 8, the circuit generally comprises a storage battery 101 which is mounted on a vehicle, an ignition coil 102, a distributor 105 and a cylinder head 113 (which forms the engine). Components which are disposed within an engine room are enclosed within a block indicated in double dot phantom line 100 while components adjacent to a driver's seat are enclosed within a block indicated in double dot phantom line 200.
The battery 101 which is located within the engine room has its negative terminal connected to the body of the vehicle, which serves as a ground, while its positive terminal is connected through a supply line La to contact B of an ignition key cylinder 201 which is arranged in opposing relationship with a driver's seat. The ignition key cylinder 201 comprises a rotary switch including contact B and another contact IG, and the circuit connection therebetween may be closed or opened by inserting and turning a key 202 which is specifically related to the particular ignition key cylinder 201. It will be appreciated that the key 202 is inherently related to a particular vehicle and also serves as a door lock key. In the description to follow, a switch comprising contacts B and IG will be referred to as an ignition switch. The contact IG is connected to the primary coil 1 of the ignition coil, which is also within the engine room, through the supply line La. In this manner, the supply line La which provides a connection between the positive terminal of the battery 101 and the primary coil 1 extends to the driver's seat once where the ignition switch is connected therein.
The other end of the primary coil 1 is connected through a ground line Le to a breaker arm 107 of the distributor 105. The distributor 105 comprises a breaker plate 109 including a cam 106, the breaker arm 107 and a point arm 108, and a rotor head 112 including a rotor 110 and a plurality of segments 111a, 111b, 111c and 111d. As indicated by broken lines, the rotor 110 and the cam 106 are coupled together through a drive shaft, not shown. This drive shaft is coupled to the crankshaft of the engine and thus rotates together with the engine. Both the breaker arm 107 and the point arm 108 are provided with contact points, and the point arm 108 is connected through the ground line Le to the ground or the body of the vehicle. In this manner, the contact points are connected in the ground line Le which provides a connection between the primary coil 1 and the body of the vehicle which serves as the electrical ground.
When the ignition switch is turned on and the contact points are closed, a closed circuit is completed through a path starting from the positive terminal of the battery 101 and including the contact B and contact IG of the ignition key cylinder 201, primary coil 1, breaker arm 107, point arm 108, the body of the vehicle and returning to the negative terminal of the battery 101. As a consequence, the primary current flows through the closed circuit, whereby the primary coil 1 produces a magnetic flux. Under this condition, when the cam 106 rotates to push up the breaker arm 107 to open the contact points, the primary current is interrupted, rapidly reducing the magnetic flux developed by the primary coil. A secondary coil 2 is magnetically coupled to the primary coil 1, and accordingly, the rapid change in the magnetic flux induces a high voltage or spark voltage across the secondary coil 2. The high voltage induced across the secondary coil 2 is applied to the rotor 110 which then distributes the high voltage to individual segments 111a, 111b, 111c or 111d at predetermined times during the rotation thereof. The segments 111a, 111b, 111c and 111d are electrically connected to spark plugs 114a, 114b, 114c and 114d, respectively, of the cylinder head 113. Upon application of the high voltage to each of the spark plugs 114a to 114d, it produces a spark discharge which ignites a gas mixture within a cylinder chamber, not shown, in which the respective spark plug is disposed. Such function will hereafter be referred to as that "the engine is ignited". In FIG. 8, a capacitor C is connected across the contact points to prevent sparks from occurring as a result of a chattering of the breaker arm 107.
In the ignition circuit of the type described, the supply line La which provides a connection between the positive terminal of the battery 101 and the primary coil 1 of the ignition 102 extends from the engine room into the driver's seat once where the ignition switch is connected therein. Accordingly, it is a simple matter to pull out the terminals of the supply line La which are connected to the contacts B and IG from the rear side of the ignition key cylinder 201 or on the side opposite from the side in which the key is inserted. This means that a direct connection of the primary coil 1 to the positive terminal of the battery 101 is enabled by pulling out the supply line La to short-circuit the ignition switch without requiring the key 202. In other words, a conventional ignition circuit suffers from the inconvenience that the vehicle may be subject to a theft by third party who is different from the proper owner of the inherent vehicle key, by allowing the engine to be ignited through short-circuiting the ignition switch thereof.